An operator of large Liquid Petroleum Gas (LPG) storage spheres first alerted HSE to the problems of Hydrogen Fabrication Cracking (HFC). During a routine inspection, a small weld repair was necessary, and during this procedure, large embedded defects were discovered. The HSE and the operator carried out a detailed investigation into the cracking on two spheres. In one sphere, 46% of the total weld length had to be repaired. Samples taken confirmed that the defects were HFC and had occurred during original manufacture. Further investigation identified that in the UK, a total of 25 LPG spheres had hydrogen related cracking. These vessels had been manufactured by three different organisations, using two design codes and two materials. The HSE's concerns were the condition of the other spheres in similar service, and how vessels with 100% manufacturing inspection could enter service with such large defects. A research program was set up with Serco Assurance to investigate the generation of HFC defects and to assess the reliability of NDT techniques to detect the defects. Two types of samples were manufactured, the first having a high degree of constraint and moisture ingress. These samples cracked in a manner similar to solidification type cracking. Further samples were manufactured with lower levels of constrain and water ingress, and embedded HFC defects were generated that were identical to those found in the original spheres. It was found that only two faulty weld runs in a multi pass weld could generate HFC that could impair the total integrity of the weld. Radiography and ultrasonic inspection, representative of the procedures applied during vessel manufacture, was carried out on both types of samples. Radiography detected the solidification type cracking, but not for the full length of the defect, but did not detect any of the HFC defects. Ultrasonic inspection detected all defects, and these results were confirmed by sectioning the samples. Modelling techniques were applied to the radiography technique to evaluate the experimental results. Modifications were made to the samples and radiography technique to evaluate possible improvement of the detection rate. Recommendations on inspection techniques to detect this type of defect are given.
Hydrogen fabrication cracking: HSE's industrial experience, and research into defect detection with a number of ndt techniques
2004
14 Seiten, 5 Bilder, 4 Tabellen, 7 Quellen
Aufsatz (Konferenz)
Englisch
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